While it is generally recognized that perceived brightness is not a point-by-point representation of retinal illumination, there is no consensus about how brightness is computed. The goal of the proposed research is to answer the following questions, thus clarifying the mechanisms by which the brain determines the brightness of objects. 1. What function of a luminance distribution determines brightness (luminance itself, contrast at edges, contrast at all spatial scales)? Do edges play a special role in determining brightness? What is the neural process or representation that makes a uniformly reflective area appear to have uniform brightness? 2. What are the temporal properties of brightness? Does brightness diffuse? 3. With certain types of luminance modulation, brightness at the center of areas of uniform luminance appears to lag behind changes at the edge. Is this because of the limited rate of brightness diffusion? Is it based on slower temporal responses of low spatial frequency filters? 4. It appears that as squarewaves increase in size, there is a preferential loss of sensitivity to low spatial frequencies. As this isn't because of the contrast sensitivity function, does it mean that there are frequency interactions biased toward low frequencies? Are these interactions simply based on frequency or do they depend on higher order structure such as the phase alignment of components in the squarewave? 5. Are frequency specific interactions significantly involved in gradient brightness illusions such as the Craik-O'Brien-Cornsweet effect? This work is envisioned as part of a larger investigation with the goal of understanding how the brain represents and processes information about the surface properties of objects. Because of the close similarities between brightness and color perception (e.g. constancy, contrast illusions, assimilation), new insights about brightness provided by this study will be considered for their possible applicability to understanding mechanisms of color vision. Another longer range goal of the investigation is to use the same stimuli from the psychophysical studies in physiological experiments to assess the cortical representation of brightness.